When objects such as zoonotic parasites are found in sections of surgical or autopsy tissue there is always interest and often advantages in making exact identifications of them. As such identifications usually are inexact, and often are incorrect, neither the range or frequency of occult parasitic infection in man is well documented. However, the feasibility of making such identifications has been demonstrated by published descriptions of several representative types. Moreover, the non-inflammatory lesions (tumors) induced by parasites are often specific in type. The objectives in this study are 1) to describe and identify a number of parasites that have been found in human tissues, 2) to develop a system of differential diagnosis based on a) the microstructure of parasites in tissue sections and b) the characteristic features of lesions induced by them, and 3) to describe the patterns of behavior and development of certain species of zoonotic parasites so that their migratory behavior and localization in human tissues can be anticipated and interpreted.